NOx in atmosphere causes photochemical smog or acid rain. Therefore, emission of NOx from, for example, an automobile equipped with an internal combustion engine, which is one of NOx generation sources, has become a social problem. For this reason, investigation has been progressed in a direction of making a law and regulations on the emission amount of NOx severer in the future. However, since a diesel engine or a gasoline lean-burn engine performs lean combustion in which a large amount of oxygen exists, reduction and removal of NOx is difficult.
For this reason, various catalysts for NOx purification have been conventionally proposed. For example, a NOx storing treatment method is mentioned which is represented by a method of storing nitrogen oxide, then introducing hydrocarbon (HC) that is a reducing substance by engine control, and removing the nitrogen oxide (Patent Literature 1). Further, as a technique of compensating for drawbacks in the storage and removal of NOx, a method is mentioned in which a catalyst oxidizing HC and CO is disposed at the upstream side of the flow of an exhaust gas and a catalyst for NOx purification is disposed at the downstream side (Patent Literature 2). In addition, there is also a method aiming at effectively utilizing H2 contained in an exhaust gas, in which HC and CO are selectively purified at the upstream side and NOx is reduced at the downstream side by using the remaining H2. Furthermore, a technique is proposed in which a reducing agent such as ammonia is actively introduced into an exhaust gas so that NOx is reduced and removed (Patent Literature 3).